This invention relates to a lash adjuster for automatically adjusting the valve clearance of a valve gear in an internal combustion engine.
In a valve gear for opening and closing an intake valve or an exhaust valve (hereinafter simply referred to as a valve) by rotating a cam, a valve clearance is automatically adjusted by means of a lash adjuster.
As a lash adjuster of this type, the present applicant has already proposed a thread type lash adjuster which can reduce the axial length of the valve gear, is small in the number of parts and easy to assemble (JP patent publication 11-62519).
With the lash adjuster of this publication, as shown in FIG. 14, a nut member 8 is fixed e.g. by brazing to the bottom surface of an end plate 2 of a lifter body 1. A threaded hole 3 is formed in the nut member 8. An adjust screw 4 threadedly engaging in the threaded hole 3 is axially biased by an elastic member 5 mounted in a closed end of the threaded hole 3. Female threads 3a of the threaded hole 3 and male threads 4a formed on the adjust screw 4 are serration-shaped so that the flank angle of pressure flanks 6, which bear push-in loads applied to the adjust screw 4, is larger than the flank angle of clearance flanks 7. The serrated threads have such a lead angle that under the bias of the elastic member 5, the adjust screw 4 will move in the axial direction while turning.
The lash adjuster having such a structure is, as shown in FIG. 14, mounted between a cam 10 and a valve stem 12 provided on a valve 11 to press the end face of the valve stem 12 against the end face of the adjust screw 4 by the bias of a valve spring 13 for biasing the valve stem 12 toward the cam 10 and serves to transmit the pressing force to the cam 10 through the lifter body 1 so that the valve 11 will be opened and closed as the cam 10 rotates.
In mounting such a lash adjuster, if a valve clearance is produced between the valve stem 12 and the adjust screw 4, by the biasing force of the elastic member 5, the adjust screw 4 will move in the axial direction along the clearance flanks 7 while turning, thus absorbing the valve clearance.
Also, when the adjust screw 4 receives a push-in force from the valve stem 12, it will retract until axial clearances formed between the female threads 3a and the male threads 4a disappear. When a further push-in force is applied, it will be borne by the pressure flanks 6, which prevent the adjust screw 4 from retracting while turning.
If the push-in load applied from the valve stem 12 to the adjust screw 4 is a continuously acting varying load, the adjust screw 4 will retract axially while turning to a position where the minimum value of the varying load becomes zero, thereby keeping uniform the valve clearance.
With the already proposed lash adjuster, when the push-in force is applied to the adjust screw 4, the circumferential component of the vertical resistance produced between the thread surfaces gives the adjust screw 4 a turning force.
At this time, if the frictional force working between the thread surfaces is large enough to give a sufficient turning resistance, the adjust screw 4 is prevented from retracting. But if the frictional force is insufficient, the adjust screw 4 will retract while turning, so that the valve lifting amount decreases. If the retracting amount exceeds the ramp of the cam 10, the valve 11 will get impulsively seated on the valve seat 18, producing an abnormal sound.
The surface roughness of the pressure flanks 6 of the female threads 3a and the male threads 4a greatly influences the frictional force between the thread surfaces. Generally, for oil-lubricated frictional surfaces, the larger the surface roughness, the higher the friction coefficient. Thus, with a lash adjuster as described above, it is preferable that the surface roughness of the female threads 3a and the male threads 4a is large.
Mass-produced screws are typically manufactured by rolling in view of lower cost and high strength. The surface roughness of flanks (thread surfaces) of screws formed by rolling is relatively small.
The inventors of the present invention formed the male threads 4a of the adjust screw 4 of the lash adjuster by rolling, and measured the valve lifting amount in a revolving number sweeping test for a lash adjuster in which is mounted this adjust screw 4. The test results are shown in FIG. 5. The surface roughness Ra of the flanks 6 and 7 of the male threads 4a of the adjust screw 4 was 0.1.
In the graph of FIG. 5, the bent line A1 at the lower part of the graph shows the number of revolutions of the crankshaft. It linearly increases from the idling revolution 800 (r/min) to MAX 6000 (r/min) and linearly decreases back to 800 (r/min).
The upper part of the graph shows a lifting curve B1 of the valve 11. In the graph, only one lifting curve is shown enlarged. But actually, such lifting curves appear continuously in the direction of the horizontal axis (that is, time axis) of the graph. The density of lifting curves are coarse in a region where the number of revolutions of the crankshaft is low and it increases as the number of revolutions of the crankshaft increases. Displaying such lifting curves is difficult. Thus, they are shown with the valve closed positions and the valve open positions connected by lines. In the graph of FIG. 5, the upper line C shows the valve closed position and the lower line D shows the valve open position.
As is apparent from the test results, if the surface roughness of the threaded surfaces of the male threads 4a of the adjust screw 4 is relatively smooth, under some conditions of the lubricating oil, a sufficient frictional force would not be obtained between the flank surfaces, so that stoppage upon loading is delayed momentarily, thus reducing the valve lifting amount.
An object of this invention is to provide a lash adjuster in which the valve clearance is automatically adjusted by axial movement of the adjust screw having serrated male threads, and the valve lifting amount is kept constant by providing a sufficient frictional resistance between the thread surfaces during loading for the adjust screw to stop.
With the lash adjuster mounted in a direct type valve gear shown in FIG. 11, due to displacement of the relative position of the cam 10 and the lifter body 1 the lifter body tends to turn under a turning force by contact with the cam 10. Also, the valve 11, too, tends to turn according to operating conditions. These rotations are not constant in the rotating direction and rotating speed. When such a rotation occurs, the thread engagement portions between the threaded hole 3a and the adjust screw 4 are acted by torsional moment produced due to a relative turning between the lifter body 1 and the valve 11.
If such a torsional moment is relatively small, even if it is a turning force in such a direction as to push in the adjust screw 4, frictional force between the pressure flanks 6 will prevent the adjust screw from slip-turning.
On the other hand, the torsional moment applied to the thread engagement portions between the adjust screw 4 and the threaded hole 3a depends on the relative turning speed of the lifter body 1 and the valve 11 and the frictional force between the end faces of the valve stem 12 and the adjust screw 4, so that its intensity and direction vary according to conditions such as the kind of engine, the number of cylinders or the number of revolutions.
If the torsional moment should be inputted at such an intensity as to overcome the frictional force between the pressure flanks 6 in such a direction as to push in the adjust screw 4, the adjust screw would retract, thereby varying the valve lifting amount. If the retracting amount exceeds the ramp height of the cam 10, the valve 11 may be impulsively seated on the valve seat 18, thus producing abnormal sound.
The second object of this invention is to provide a lash adjuster which can maintain a stable valve lifting amount under any conditions, irrespective of the relative turning speed between the lifter body and the valve stem or the frictional coefficient of the abutting portions, and which can be easily mounted in a valve gear.
To achieve the first object, according to this invention, there is provided a lash adjuster for use in a valve gear, comprising a lifter body having an end plate, mounted between a cam and a valve stem and supported so as to be slidable in the axial direction, an adjust screw in threaded engagement with a threaded hole formed in the bottom surface of the end plate of the lifter body, and an elastic member biasing the adjust screw toward the top end of the valve stem, wherein threads of female threads of the threaded hole and male threads formed on the outer periphery of the adjust screw are serrated so that the flank angle of pressure flanks of the male threads and the female threads, which receive axial push-in force applied to the adjust screw, is larger than the flank angle of clearance flanks of the male threads and the female threads, wherein the surface roughness Ra of the pressure flanks of at least one of the female threads and the male threads is set to 0.4 or over.
With this arrangement, it is possible to obtain a sufficient frictional force between the pressure flanks of the female threads of the threaded hole and the male threads of the adjust screw. Thus, even when an axial push-in load is applied from the valve stem to the adjust screw, the adjust screw will not retract while turning. Thus, the valve lifting amount varies little, so that it is possible to obtain stable valve lifting properties.
The pressure flank having the surface roughness Ra of 0.4 or over may be at least one of the pressure flanks of the male threads of the adjust screw and the pressure flanks of the female threads of the threaded hole, or both of them.
As a method of obtaining pressure flanks having a surface roughness Ra of 0.4 or over, the threads may be formed by cutting, knurling after forming the threads by rolling, or shot-peening after forming the threads by rolling.
By employing these methods, the surface roughness Ra of 0.4 or over will be obtained not only on the pressure flanks but on the clearance flanks. But, if the surfaces of the female threads and of the male threads are rougher than necessary, movement of the adjust screw toward the protruding side would worsen. While the valve clearance is increasing, trouble may occur that even when the axial load from the valve stem becomes zero, and even after the adjust screw has contacted the clearance flanks, it would not begin slip turning. In order to prevent such a problem, the surface roughness Ra of the clearance flanks is set to less than 25.
By setting the clearance flanks to less than 25, the adjust screw will move smoothly in the protruding direction. Thus it is possible to obtain smooth operating properties.
To achieve the second object, according to this invention, there is provided a lash adjuster for use in a valve gear, comprising a lifter body having an end plate and mounted between a cam and a valve stem so as to be slidable in the axial direction, a nut member fixedly mounted on the bottom surface of the end plate of the lifter body, an adjust screw in threaded engagement with a threaded hole formed in the nut member, and an elastic member mounted in the threaded hole to bias the adjust screw in the axial direction, wherein threads of female threads of the threaded hole and male threads formed on the outer periphery of the adjust screw are serrated so that the flank angle of pressure flanks of the male threads and the female threads, which receive axial push-in force applied to the adjust screw, is larger than the flank angle of clearance flanks of the male threads and the female threads, wherein a slide member is mounted between the adjust screw and the valve stem and a retaining means is provided to prevent the slide member from turning relative to the nut member while supporting the slide member so as to be movable in the axial direction.
By mounting the slide member between the adjust screw and the valve stem and preventing the slide member from turning relative to the nut member while allowing the slide member to move in the axial direction, even if the valve stem turns relative to the lifter body, no turning moment will be transmitted to the adjust screw, so that only axial push-in force is transmitted. The axial push-in force is borne by the pressure flanks at the thread engagement portions between the threaded hole and the adjust screw, so that the adjust screw is prevented from retracting while turning. As a result, it is possible to maintain a stable valve lifting amount.
By retaining the slide member by means of the retaining means, in the stage before the lash adjuster is mounted in a valve gear, it is possible to prevent the slide member from falling off the lifter body, so that it is possible to make it easy to mount the lash adjuster.
As a method of preventing the slide member from turning and making it inseparable, it is conceivable to provide a cylindrical portion on the outer peripheral portion of the nut member, form an axial groove at the bottom end of the cylindrical portion, fit a turn-preventive piece provided on the outer periphery of the slide member in this groove so that the slide member is prevented from turning but is axially movable, and prevent the slide member from turning by mounting a snap ring on the inner periphery of the lifter body.
But in this case, a problem arises that the outer peripheral wall of the nut member would be thick, thus increasing the weight of the lash adjuster.
In order to solve this problem, according to the present invention, the retaining means comprises an elastic ring having its outer peripheral portion fitted in a groove formed in the inner periphery of the lifter body, and a ring-shaped turn-preventive member pressed against the bottom surface of the nut member by the elastic ring, a plurality of guide pieces formed to extend downwardly from the inner periphery of the turn-preventive member, the guide pieces each being formed with a guide hole, the slide member having a plurality of turn-preventive pieces formed on the outer periphery thereof, the turn-preventive pieces being inserted in the guide holes to prevent the slide member from turning and support the slide member so as to be movable in the axial direction.
By employing the retaining means, it is possible to form the turn-preventive member by pressing a thin metal plate. Thus it is possible to suppress increase in the weight of the lash adjuster to a minimum and to provide a lightweight, low-cost lash adjuster.
As the elastic ring, a disc spring having a cut-off portion in its circumference may be used. This elastic ring is easy to mount and has not only the function as a spring, but also the function as a snap ring. Thus, it is not necessary to use a separate snap ring to prevent the elastic ring from coming off. Thus it is possible to reduce the number of parts, thereby reducing the cost.
By forming a plurality of protrusions on the outer periphery of the turn-preventive member and coupling the turn-preventive member to the nut member by caulking the protrusions, it is possible to reliably prevent the turn-preventive member from turning. Thus it is possible to more reliably prevent the slide member from turning.
Further, by mounting the nut member in contact with the bottom surface of the end plate of the lifter body and pressing it against the bottom surface of the end plate with the elastic ring, it is possible to prevent the nut member from turning by the bias of the elastic ring. Thus, compared with the case in which the nut member is fixed to the bottom surface of the end plate of the lifter body by brazing, it is possible to reduce the cost of the lash adjuster.
In the lash adjuster of the present invention, the retaining means may comprise an elastic ring mounted in a groove formed in the inner periphery of the lifter body for pressing the nut member against the bottom surface of the end plate of the lifter body, the nut member being formed with cutouts in an outer periphery thereof at opposed positions, the slide member having L-shaped turn-preventive pieces adapted to be fitted into the cutouts formed in the nut member and prevented by the elastic ring from coming out of the cutouts.
By adopting such a retaining means, the turn-preventive member of the abovesaid embodiment is not needed any more. This reduces the number of the parts of the lash adjuster and its cost and weight.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: